1. Field of the Invention
The invention relates to a force sensing element and, more particularly, to a force sensing element that detects a load or the like by making use of a piezoresistance effect of a semiconductor for converting a compressive force into an electric signal.
2. Description of the Related Art
Heretofore, as a force sensing element used to detect a combustion pressure in a cylinder of an engine or the like, a compact force sensing element employing a piezoresistance element has been known. If a strain (deformation under stress) occurs in a certain portion of the piezoresistance element, the portion changes in resistivity in accordance with a magnitude of the strain. This piezoresistance element is generally constructed of a gauge resistance that is formed on a main face of a single-crystal Si substrate by means of a semiconductor manufacturing technique.
One known strain gauge employing a piezoresistance element is constructed as follows. An electrode is formed on a lateral face of the strain gauge or on another face thereof, which faces a stress-working face thereof. An expandable object to be measured is stuck on the stress-working face. A stress is detected from a current that changes in accordance with expansion of the object to be measured. In this strain gauge, since a stress is applied in the same direction as a current flows, a piezoresistance coefficient π11 is utilized.
Further, one known force sensing element is constructed as follows. A Wheatstone bridge is constituted by forming four gauge resistances demonstrating a piezoresistance effect on an Si substrate whose (110)-face serves as a main face in such a manner as to extend in <100>- and <110>-directions. A force transmission block is disposed on this Wheatstone bridge to construct a force sensing element (which is disclosed, for example, in Japanese Patent Application Laid-Open No. 8-271363). In this force sensing element, if a force is applied to the force transmission block, a stress is transmitted from the force transmission block to a thickness direction of the Si substrate. Therefore, a voltage is applied such that a current flows perpendicularly to the direction in which the stress is transmitted. The gauge resistance produces a piezoresistance effect in accordance with the stress. Because the piezoresistance effect in the <100>-direction is different from the piezoresistance effect in the <110>-direction, there is created a difference between resistance values. By detecting the difference between the resistance values as a difference in voltage, a force transmitted to the force transmission block is detected.
In this force sensing element, a voltage is detected in the same direction as a current path extends, and a force (a uniaxial stress) is applied perpendicularly to the current path. Therefore, a piezoresistance coefficient π13 is utilized.
In the case where this force sensing element is used as a combustion pressure sensor, the force sensing element is disposed at the center of a sensor housing and packed, and electrodes of the force sensing element are respectively connected to fetch terminals through wire bonding. The fetch terminals are formed coplanar with the electrodes respectively. A force transmission rod and a diaphragm are provided in an upper portion of the force sensing element in such a manner as to contact the force transmission block thereof. A force applied to the diaphragm is transmitted to the force sensing element via the force transmission rod. Thus, as described above, a combustion pressure is detected from a difference in voltage.
The force transmission block disposed on the piezoresistance element is composed of a high-rigidity rectangular parallelepiped force transmission body and a semispherical force transmission body disposed on the rectangular parallelepiped force transmission body, in consideration of characteristics of force transmission to the piezoresistance element (e.g., disclosed in Japanese Patent Application Laid-Open No. 2001-305001). In this force transmission block, the semispherical force transmission body converts a concentrated load applied to an upper semispherical portion thereof into a uniformly distributed load in a lower face thereof. This load presses and deforms the gauge portion of the piezoresistance element via the rectangular parallelepiped force transmission body.
However, the strain gauge of the aforementioned related art requires sticking the gauge on an object to be measured by means of an adhesive or the like. Therefore, dispersion of thickness, location, or the like of the adhesive causes a problem in that measurement results cannot be obtained with high precision. Also, at a high temperature above 400° C., the adhesive does not function sufficiently and makes the implementation of measurement impossible.
In the force sensing element of the related art, since the electrodes are formed on the same plane, wiring for outputting a gauge voltage needs to be created through wire bonding. This causes a problem of an increase in the number of manufacturing processes and a problem of difficulty in manufacturing the force sensing element at a low cost.